Allow mid-session model switching with a persisted transcript marker

Picker stays live for the session; switches persist a model_switch notice (§17 revised).
Rebinds refused mid-turn; images become placeholders for non-vision targets at send time.

surfaces/gui/e2e/model-switch.spec.ts

@@ -0,0 +1,33 @@
+// Model-layer roadmap item 3 (2026-07-22): the model picker stays actionable for the
+// session's whole life (supersedes the 2026-07-04 lock that hid it after the first turn).
+// A mid-session switch drops a persisted info marker into the transcript, and later
+// messages ride the new model.
+import { expect } from "@playwright/test";
+import { test } from "./fixtures";
+
+test("mid-session model switch shows the marker and later turns use the new model", async ({
+  page,
+}) => {
+  await page.goto("/");
+  await page.getByText("Draft the launch note").first().click();
+  const box = page.getByPlaceholder(/Ask the coworker/);
+  await box.fill("hello there");
+  await box.press("Enter");
+  await expect(page.getByText("Echo: hello there", { exact: false }).first()).toBeVisible();
+
+  // The picker is still in the composer after the first turn (the old lock hid it).
+  const picker = page.locator(".dd").filter({ hasText: "Claude Opus 4.8" });
+  await expect(picker).toBeVisible();
+  await picker.locator(".pill").click();
+  await page.locator(".dd-item").filter({ hasText: "GPT-5.5" }).click();
+
+  // The switch marker lands in the transcript…
+  await expect(page.getByText(/Model switched to gpt-5.5/).first()).toBeVisible();
+
+  // …and the next message carries the new model (the fixture echoes it back).
+  await box.fill("after the switch");
+  await box.press("Enter");
+  await expect(
+    page.getByText("Echo: after the switch [model=gpt-5.5]", { exact: false }).first(),
+  ).toBeVisible();
+});